An electrophotographic image forming apparatus forms an electrostatic latent image on a photoconductor by optical writing and develops the electrostatic latent image to obtain a toner image. The toner image is transferred to a sheet and is fixed to the sheet through heat, pressure, or the like, thereby forming an image on the sheet.
In full-color image forming apparatuses, a technique is known that transfers a toner image to an intermediate transfer body and transfers the transferred toner image from the intermediate transfer body to a sheet. A color toner image is, for example, once transferred (primarily transferred) to the intermediated transfer body such as an intermediate transfer belt and an intermediate transfer drum. After color toner images of a plurality of colors are superimposed on the intermediate transfer body, the color toner images are transferred (secondarily transferred) from the intermediate transfer body to the sheet. Subsequently, the color toner images on the sheet are fixed, thus achieving a full-color image.
In the image forming apparatus using such an intermediate transfer body, speed fluctuations occurring in the intermediate transfer body cause misregistration of the color toner images during the primary transfer. The misregistration of the color toner images can cause color irregularities or streaky images, thus reducing image quality.
In order to suppress the speed fluctuations of the intermediate transfer body, rotary eccentricity has been conventionally suppressed by increasing the accuracy of machining shafts and motors or by using high-precision gears. In control systems, two sensors are used to suppress mounting eccentricity of a rotary encoder of a rotary detection system, or a gain of a control circuit is increased to increase detection sensitivity, thereby suppressing rotary fluctuations. These measures can effectively suppress the speed fluctuations of the intermediate transfer body caused by periodic stationary disturbances.
The speed fluctuations of the intermediate transfer body can also occur by a sheet rushing into a nip in between the intermediate transfer body and a roller in which the secondary transfer is performed. When the sheet enters the nip, for example, a load of the roller increases or decreases, and a transfer speed of the intermediate transfer body momentarily slows or decreases. In other words, the speed fluctuations of the intermediate transfer body occur.
The sheet rushing is a non-stationary, transient disturbance and has broad frequency characteristics. The above-described measures have difficulty in effectively suppressing the speed fluctuations of the intermediate transfer body caused by such a non-stationary disturbance.
The disturbance caused by the sheet rushing may not only cause the speed fluctuations of the intermediate transfer body but also vibrate an optical writing unit or a primary transfer unit through a casing. Such vibration causes a reduction in image quality.
In Japanese Laid-open Patent Publication No. 2008-139749, in order to suppress load fluctuations and speed fluctuations of an intermediate transfer belt caused by sheet passing, load facing rollers that nip the intermediate transfer belt are provided. By giving a rotational load to the load facing rollers in advance, an influence of the load fluctuations is reduced not only at the time of sheet passing. The load facing rollers are given a rotational load by a sliding abutment member and are brought into pressing against the intermediate transfer belt by springs.
In Japanese Patent No. 4807752, a pressurizing roller and a heat roller nip a recording medium therebetween, and an actuator controls a pressurizing force of a spring applied to the pressurizing roller. The actuator adjusts the distance between a holding member that holds the pressurizing roller and the spring and the heat roller, thereby changing the pressurizing force of the spring.
In Japanese Laid-open Patent Publication No. 2008-139749, although positions of the rollers are controlled by a cam, a pressing force of the rollers by the springs is not controlled. Given this configuration, owing to fluctuations of the shape and thickness of sheets, for example, force can abruptly acts on the intermediate transfer belt.
In Japanese Patent No. 4807752, although the actuator controls a position of the holding member, the actuator does not directly control a pressurizing force of the heat roller. Given this configuration, adjustment of the pressurizing force of the heat roller is restricted to be within a movable range of the spring. Furthermore, it is difficult to increase responsiveness about controlling the pressurizing force of the heat roller.
Therefore, there is a need to set force acting on a part that can come close to or separate from an object with high precision and good responsiveness.